Bovine thrombin is being prepared by activation of purified prethrombin with the use of a five-component system consisting of purified Ac-globulin, "cephalin" autoprothrombin C, and calcium ions. Crystals obtained from this preparation are to be studied by x-ray diffraction. A protein representing the difference between prethrombin (Lys) and prethrombin (Thr) is to be isolated and work on the primary structure is to be done. Likewise, a portion of the prothrombin molecule from the N-terminal end to the beginning of prethrombin (Lys) is to be isolated and studied from the viewpoint of primary structure. Autodigestion products of thrombin are also to be prepared. The various derived proteins are to be studied by immunology techniques to find antigenic determinants and recognition qualities against other components of the prothrombin complex, and derivatives of the complex.